This study aims to explore the dryout characteristics of two-phase closed thermosiphon. It involves the design and experimentation on low-liquid filled heat pipes. We investigated the effects of different fill ratios, heating water temperatures, and cooling water flow rates on the start-up characteristics of two-phase closed thermosiphon. Based on the network model, we proposed an analytical expression for the thermal resistance, R, of two-phase closed thermosiphon, and provided an explanation of the various thermal resistance components. This research expands our understanding of dryout and provides insights for optimizing heat pipe design and addressing heat conduction issues with low fill ratios.